Re: reordering of trapping operations and volatile

[Martin Uecker <ma.uecker@gmail.com>]

Am Samstag, den 08.01.2022, 10:35 -0800 schrieb Andrew Pinski:
> On Sat, Jan 8, 2022 at 12:33 AM Martin Uecker via Gcc <gcc@gcc.gnu.org> wrote:
> > 
> > Hi Richard,
> > 
> > I have a question regarding reodering of volatile
> > accesses and trapping operations. My initial
> > assumption (and  hope) was that compilers take
> > care to avoid creating traps that are incorrectly
> > ordered relative to observable behavior.
> > 
> > I had trouble finding examples, and my cursory
> > glace at the code seemed to confirm that GCC
> > carefully avoids this.  But then someone showed
> > me this example, where this can happen in GCC:
> > 
> > 
> > volatile int x;
> > 
> > int foo(int a, int b, _Bool store_to_x)
> > {
> >   if (!store_to_x)
> >     return a / b;
> >   x = b;
> >   return a / b;
> > }
> > 
> > 
> > https://godbolt.org/z/vq3r8vjxr
> 
> The question becomes what is a trapping instruction vs an undefined
> instruction?
> For floating point types, it is well defined what is a trapping
> instruction while for integer types it is not well defined.
> On some (many?) targets dividing by 0 is just undefined and does not
> trap (powerpc, aarch64, arm and many others; MIPS it depends on the
> options passed to GCC if the conditional trap should be inserted or
> not).

> The other side is if there is undefined code on the path, should
> observable results happen first (stores to volatile/atomics, etc.)?

I think for volatile stores and I/O, I think it would be
nice of we could guarantee that those happen before the UB
ruins the day. (I am not sure about atomics, those are 
not directly obsevable)

For I/O this is probably already the case (?).
For volatile, it seems this would need some tweaks.

I am trying to figure out whether this is feasible.

> GCC assumes by default that divide is trappable but stores not are not
> observable. This is where -fnon-call-exceptions come into play.

Ok, thanks! I will look at this!

> In the second case, GCC assumes reducing trappable instructions are
> fine. 

-fnon-call-exceptions would treat trapping instructions
as defined (and trapping) instead of UB? This is
then probably even stronger than the requirement above.

> Note I thought -fno-delete-dead-exceptions would fix the sink
> but it didn't.


Martin